Many communities offer members incentives to join. Sometimes those incentives work and lots of people join. But it’s a terrible idea. It’s very difficult to convert incentivised members into regular members.

This is because incentives are rewards for actions. This means instead of a member doing something because they enjoy it or because it fits with their self concept (“I’m the sort of person that would join a community like this”) they attribute the action to the reward (“I joined the community to get the reward”). In future, no reward = no action.

The former will continue to participate because believe they are the sort person who joins these communities. The latter will wait for more rewards. If, like most people, you don’t have an unlimited number of rewards, you’re going to find it difficult to motivate members to take action.

The challenge then becomes not in persuading members to join the community for an external reward but to persuade potential members that they are the sort of people that would join a community like yours. It's harder to achieve, but more much effective.